impedance matching for a helix

[ruchak]

hi, i am trying to make a helix in SINGULA software. i have succeeded partly. but i am not getting satisfactory results(the points in the smith chart do not fall within the 1db circle:sad as i am unable to implement impedance matching technique. please help me to design a 3 turn helix for 2.4GHz.

 

[elchiquito]

Input Imedance of an axial mode helix antenna with a ground which is greater than one wavelength is almost resistive with values between 100 and 200 ohms. the estimated value with 20 percent accuracy is given by the equation R=140C/lambda

C= Circumference of the antenna.

A suitable matching circuit is required by the antenna if it is to be connected to a 50 ohm transmission line. one way to do it is to use the first 1/4 turn of the helix as an impedance transformer. a microstrip is an ideal structure for this purpose. since the ground plane may be the same as that of the antenna. the dielectric substrate height h is linked to the strip width w and feed line impedance Z0 by the equation

h=(w)/((377/sqrt(espilsonr)*Z0)-2)

Hope that helps

Regards
Elchiquito

 

[ruchak]

thanks it helped

 

[DeboraHarry]

I noticed on the antenna design kit for HFSS, one can design axial mode helical antennas which have a good return loss at 50 Ohms. It appears to achieve this by using a very large diameter conductor (~0.056λ) . I'm not convinced that is an optimal method, but I thought it worth mentioning.

 

[ferdows]

antenna design kit is not trusty software u can find some helical calculator in web

 

[DeboraHarry]

antenna design kit is not trusty software u can find some helical calculator in web

What do you mean by "not trusty"? Do you have specific examples of where it is bad?

- - - Updated - - -

hi, i am trying to make a helix in SINGULA software. i have succeeded partly. but i am not getting satisfactory results(the points in the smith chart do not fall within the 1db circle:sad as i am unable to implement impedance matching technique. please help me to design a 3 turn helix for 2.4GHz.

Take a look at this paper:

"A 50-ohm input impedance for helical beam antennas"
Antennas and Propagation, IEEE Transactions on
Date of Publication: Nov 1977
Author(s): Kraus, J.
Ohio State Univ. Radio Obs., Columbus, OH USA
Volume: 25 , Issue: 6
Page(s): 913

It shows a way to achieve 50 Ohms. Basically Kraus puts a wider strip of material on the lowest quarter turn, to form a lower impedance transmission line, so it matches the impedance to 50 Ohms.

I'd considered using a quarter wave of 75 Ohm coax, but that is not a perfect match - ideally you need a transmission line around sqrt(140*50) - whatever that happens to work out to. But putting a wider strip seems a good idea to me. Who would argue with Kraus?

 

[ruchak]

i tred using HFSS also..i suceeded a li'l trying triangular strip. but i dont know what should be the length,breadth and width of traingular strip. are there some design parameters as to what should be the length and width of triangular strip with varying height of helix.is there any relation?
and to make it conformal, i inserted a cavity for each helix in a 2x2 helix array. so the capacitance of the total antenna increased. Again a problem.

 

[DeboraHarry]

i tred using HFSS also..i suceeded a li'l trying triangular strip. but i dont know what should be the length,breadth and width of traingular strip. are there some design parameters as to what should be the length and width of triangular strip with varying height of helix.is there any relation?
and to make it conformal, i inserted a cavity for each helix in a 2x2 helix array. so the capacitance of the total antenna increased. Again a problem.

I'd guess you want to make the line sqrt(140*50)=83 Ohms. As you move further from the ground plane, the line would need to get wider to keep 83 Ohms. So that might be one way to approach it. Keep the W/h such that the Zo is always the above value.

At the end of the day, it will need a bit of tweaking.

There might be some theoretical advantage in having Zo of the line taper from 50 to 140 Ohms. I think someone has shown the optimal taper for maximum bandwidth. You could try to change W/h along the line, so that you gradually change the line impedance.